Coronal Magnetic Implosions

Science Nugget: Sep 24, 1999

Disclaimer

We (the SXT Chief Observers) usually give you pretty pictures
in these science nuggets, and often offer
some half-baked explanation for what we are seeing.
It needs to be half-baked because we're really writing a science newspaper
here, rather than entombing the results in a regular archival research
paper.
Also, as observers, we are not really so good at theoretical work (but
some SXT chief observers also wear theorists' hats at times!).
However -- until you have a correct theory for something,
you don't understand it properly.
So this week's science nugget takes the opposite tack from some of our
earlier instrumentation-oriented science nuggets
[1][2]
and instead is
unabashedly theoretical
.

Solar magnetic explosions

We've written many science nuggets, e.g.
[1][2][3][4][5][6]
on eruptive phenomena, of various kinds, as seen by Yohkoh SXT.
Of course we can't compete in spectacular images of eruptions with
the
LASCO
instrument on SOHO, a coronagraph that can see ejecta some 20 million km
from the surface of the Sun.
But we (and other instruments on SOHO) can see the beginnings of these ejecta
much closer to the photosphere, in many cases.

The point is that we see ejecta of many kinds - things fleeing centrifugally
from the flare core - but very little evidence for inwards motions.
Why is this?
The motions that we see in the corona represent mass trapped on coronal field
lines, and almost invariably the motions - if perpendicular to the field - are
outwards.
Mulling this curious asymmetry in what is by now a very large set of
observations, it was irresistibly tempting to try to sharpen the well-known
statement of the Aly conjecture (simply put, this states that it takes energy
to "open" the solar magnetic field ... it doesn't just happen,
like falling off a log).
In this context it's puzzling because the
observed magnetic explosions obviously result from an instability, and yet
the field is going to a higher-energy state!
However, theorists are clever and most of them seem to be sure that there are
ways around this apparent paradox.
Hence a sharper tool would be helpful.

A conjecture

Why didn't we think of this before?
The solar magnetic field stores the energy to be released by a flare, CME,
or other coronal transient; the local energy density in the magnetostatic field
is given by B^2/8*pi.
Therefore the volume integral of this quantity,
evaluated before and after a transient, must decrease or else there wouldn't
have been energy for radiation, mass motions, enthalpy, potential energy,
particle acceleration, etc...
So, the "level surfaces" (3D contours) of B^2/8*pi surely must shrink inwards
in some (the energetically dominant) parts of the coronal volume.
Here we're assuming that there is no extraneous energy input from below the
photosphere or from gravitational potential energy in the corona, and that
magnetic pressure dominates.
These are all well-accepted assumptions.

The
conjecture
announced here (and now accepted for publication in a regular paper in Solar
Physics) is that this implies the inward motion of some large number of field
lines - a "magnetic implosion" that would be necessary to drive the "magnetic
explosion" (see Moore et al., ApJ 526, #1) must occur.
This is conjectural, but I think it's true and can be proven mathematically by
some theorist or other.
Alphonse Sterling points out that the universe is full of invisible implosions
leading to spectacular explosions, for example supernovae; in our case however
the corona is optically thin and we have every expectation of being able to see
the actual implosion and to learn about flare theory from it.
Note that this conjecture applies to every theory that uses coronal magnetic
fields as a source of energy - that is, to virtually every theory of flares or
CMEs!

Afterword

Sorry, no pictures this time.
Please look at some of the nuggets linked above.